1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography, such as a copying machine, printer, facsimile, and MFPs thereof. In particular, the invention relates to a color image forming apparatus of tandem type in which a plurality of image forming units are arranged in parallel so as to be opposed to an intermediate transfer belt or a transfer carriage belt.
2. Description of the Related Art
As image forming apparatuses such as copying machines and printers, color image forming apparatuses of tandem type, having an intermediate transfer belt, have heretofore been known (for example, see Japanese Patent Application Laid-open No. 2006-91459 and Japanese Patent Application Laid-open No. 2001-356547).
Four photosensitive elements (image carriers) are arranged in parallel in positions opposed to the intermediate transfer belt. Black, yellow, magenta, and cyan toner images are formed on the four photosensitive elements, respectively. The toner images formed on the photosensitive elements in the respective colors are transferred to the intermediate transfer belt in a superposed manner at the positions opposed to the intermediate transfer belt. The toner images in the plurality of colors, carried by the intermediate transfer belt, are then transferred to a recording medium as a color image.
In such an image forming apparatus, in order to clean untransferred toner and other adhering substances off the photosensitive elements (image carriers) by a cleaning device without fail, as well as to reduce wear of the photosensitive elements, cleaning blades, and the like, there has been known a technique of using lubricant supplying units which supply the photosensitive elements with a lubricant (for example, see Japanese Patent Application Laid-open No. H11-174810).
More specifically, untransferred toner remaining on the photosensitive elements after the transfer step should be all removed by cleaning blades (cleaning devices) which are in contact with the photosensitive elements. With an aging degradation (wear) of the cleaning blades due to the contact with the photosensitive elements, it has sometimes been the case that untransferred toner slips through a gap between the worn-out cleaning blades and the photosensitive elements to cause a cleaning failure.
Even if the cleaning blades are not degraded, small particle size toners and spherical toners can get into slight gaps between the cleaning blades and the photosensitive elements. The toners could eventually get through the gaps, causing a cleaning failure.
Moreover, if toner, external additives contained in the toner, or adhering substances such as paper dust get through the gaps between the cleaning blades and the photosensitive elements, those substances could firmly stick to the photosensitive elements in a film-like form to cause filming.
To address such problems, a lubricant may be applied onto the photosensitive elements. The application of the lubricant lowers the coefficient of friction on the photosensitive elements, thereby reducing degradation of the cleaning blades and the photosensitive elements. This also improves the detachability of the adhering substances adhering to the photosensitive elements, such as untransferred toner, whereby the occurrence of a cleaning failure and filming over time can be suppressed.
Specifically, Japanese Patent Application Laid-open No. H11-174810 describes a lubricant supplying unit (lubricant applying device) which includes: a brush roller that makes sliding contact with the photosensitive element (image carrier); a solid lubricant that is in contact with the brush roller; and a spring that biases the solid lubricant toward the brush roller. The brush roller rotating in a predetermined direction gradually scrapes some of the solid lubricant, and the scraped lubricant is applied (supplied) to the surface of the image carrier by the brush roller.
Japanese Patent Application Laid-open No. 2006-91459, etc. disclose a technology, in which, in a color image forming apparatus of tandem type, the outer diameter of a photosensitive element in a black image forming unit is set to be larger than that of the photosensitive elements in other image forming units so that the black image forming unit, whose use frequency is higher than those of the other color image forming units, has an extended life.
The technology of the foregoing Japanese Patent Application Laid-open No. 2006-91459 extends the life of the black image forming unit whose use frequency is higher than those of the other color image forming units by increasing the outer diameter of the photosensitive element in the black image forming unit. There has thus been a problem because the image forming apparatus becomes accordingly larger in size.
Such a problem is not limited to image forming apparatuses of tandem type that use an intermediate transfer belt, but also applies to image forming apparatuses that use a transfer carriage belt (apparatuses in which a plurality of photosensitive elements are arranged in parallel so as to be opposed to a transfer carriage belt, and toner images on the photosensitive elements are transferred to a recording medium conveyed by the transfer carriage belt in a superposed manner).